The words "coral reef " and "sunlight " are almost synonymous in the world of marine biology. That 's because most corals exist in an unusual symbiosis with microscopic plants, called zooxanthellae. The zooxanthellae provide the coral with foods resulting from photosynthesis, a process that requires light. However, recent evidence indicates that light-limited deep reefs — those located between 50- and 150-meter (m) depth, in a range also known as the “Twilight Zone " — are an important transition habitat, where many shallow-reef and deep-sea organisms coexist with each other and with species unique to this depth range.